大全169补充162老题新问

169. Because the enemy’s new ship is the quietest and it is therefore the most elusive submarine, it is being increasingly viewed by the military as a threat to security.
(A) and it is therefore the most elusive submarine, it is being increasingly viewed
(B) it is therefore the most elusive of submarines, and it has increased the view
(C) and therefore the most elusive submarine, it is being increasingly viewed
(D) and therefore it is the most elusive of submarines, there is an increasing view
(E) therefore being the most elusive of submarines, it is increasingly viewed

A前面是不是一定要加，号（逗号）呀？是不是有it is就要加，没就不加呀？

还有是不是AND后连接一个完整句子AND前就要加逗号？

A前面是不是一定要加，号（逗号）呀？

An extra comma won't make it the best choice.

是不是有it is就要加，没就不加呀？

The usage of commas is a huge topic. You can refer to the following link for more information,

http://webster.commnet.edu/grammar/commas.htm

还有是不是AND后连接一个完整句子AND前就要加逗号？

Not necessarily. For instance,

I've seen that musical before and it was horrible.

It does not make any sense to add a comma into this sentence.